Crustal Magnetization Research Articles

A Monte Carlo model of crustal field influences on solar energetic particle precipitation into the Martian atmosphere

AbstractSolar energetic particles (SEPs) can precipitate directly into the atmospheres of weakly magnetized planets, causing increased ionization, heating, and altered neutral chemistry. However, strong localized crustal magnetism at Mars can deflect energetic charged particles and reduce precipitation. In order to quantify these effects, we have developed a model of proton transport and energy deposition in spatially varying magnetic fields, called Atmospheric Scattering of Protons and Energetic Neutrals. We benchmark the model's particle tracing algorithm, collisional physics, and heating rates, comparing against previously published work in the latter two cases. We find that energetic nonrelativistic protons precipitating in proximity to a crustal field anomaly will primarily deposit energy at either their stopping altitude or magnetic reflection altitude. We compared atmospheric ionization in the presence and absence of crustal magnetic fields at 50°S and 182°E during the peak flux of the 29 October 2003 “Halloween storm” SEP event. The presence of crustal magnetic fields reduced total ionization by ~30% but caused ionization to occur over a wider geographic area.

Magnetic anomalies and model of the magnetization in the Earth's crust of circumpolar and polar the sectors of Ural region

The study of the structural features of the anomalous magnetic field for the territory of the circumpolar and polar sector of the Urals region was carried out. The anomalies of the Earth's crust layers were identified and the maps of such anomalies were created. Map of local anomalies was used for mapping basic-ultrabasic massifs in the upper parts of the foundation within the sedimentary basins. An interpretation of regional magnetic anomalies was carried out, a model structure of the Earth's crust and their parameters are based on the results of the studies along the DSS profiles. Comparison of the deep structure of the cuts produced by independent geophysical methods based on seismic and magnetic data has enabled us to share the consolidated crust into two layers with different magnetic properties. Top layer of the Earth's crust does not make a significant contribution into regional magnetic field and is characterized by a low magnetization (less than 0,3 A/m). Within this layer magnetized local sources were identified. The lower layer has greater crustal magnetization. As a result of the two-dimensional modeling of the value of the magnetization of the basalt layer of the crust is 3—4 A/m, The average depth to the top surface of the layer is 18—20 km. The resulting parameters were used for three-dimensional modeling, Model with uniform magnetization directed along the modern geomagnetic field has been considered. For the entire region it was built the upper surface of the magnetized layer, which allowed to clarify mafic layer in the space between the DDS profiles. It was found that at the Northern, Circumpolar and Polar Urals basalt layer plunged to a considerable depth of 26—30 km.

Testing the axial dipole hypothesis for the Moon by modeling the direction of crustal magnetization

AbstractOrbital magnetic field data show that portions of the Moon's crust are strongly magnetized, and paleomagnetic data of lunar samples suggest that Earth strength magnetic fields could have existed during the first several hundred million years of lunar history. The origin of the fields that magnetized the crust are not understood and could be the result of either a long‐lived core‐generated dynamo or transient fields associated with large impact events. Core dynamo models usually predict that the field would be predominantly dipolar, with the dipole axis aligned with the rotation axis. We test this hypothesis by modeling the direction of crustal magnetization using a global magnetic field model of the Moon derived from Lunar Prospector and Kaguya magnetometer data. We make use of a model that assumes that the crust is unidirectionally magnetized. The intensity of magnetization can vary with the crust, and the best fitting direction of magnetization is obtained from a nonnegative least squares inversion. From the best fitting magnetization direction we obtain the corresponding north magnetic pole predicted by an internal dipolar field. Some of the obtained paleopoles are associated with the current geographic poles, while other well‐constrained anomalies have paleopoles at equatorial latitudes, preferentially at 90° east and west longitudes. One plausible hypothesis for this distribution of paleopoles is that the Moon possessed a long‐lived dipolar field but that the dipole was not aligned with the rotation axis as a result of large‐scale heat flow heterogeneities at the core‐mantle boundary.

Martian magnetism with orbiting sub-millimeter sensor: simulated retrieval system

Abstract. A Mars-orbiting sub-millimeter sensor can be used to retrieve the magnetic field at low altitudes over large areas of significant planetary crustal magnetism of the surface of Mars from measurements of circularly polarized radiation emitted by the 368 GHz ground-state molecular oxygen absorption line. We design a full retrieval system for one example orbit to show the expected accuracies on the magnetic field components that one realization of such a Mars satellite mission could achieve. For one set of measurements around a tangent profile, we find that the two horizontal components of the magnetic field can be measured at about 200 nT error with a vertical resolution of around 4 km from 6 up to 70 km in tangent altitude. The error is similar regardless of the true strength of the magnetic field, and it can be reduced by repeated measurements over the same area. The method and some of its potential pitfalls are described and discussed.

Crustal Magnetization of Mars: Terra Meridiani and Terra Sirenum

We examine the crustal magnetization of Terra Meridiani and Terra Sirenum, the region representing the strongest magnetization in the Southern Hemisphere, by downward continuing mapping level data (400 km altitude) from the Mars Global Surveyor (MGS) Magnetometer and Electron Reflectometer (MAG/ER). We find that the surface magnetization in both regions can be fit with a small number of sources, with the positive sources stronger than negative ones in both regions. The ratio of the strongest positive to strongest negative source for the regions matches within 2%. For both regions, the locations of strong sources are positioned at the outer rings of ancient impact features. We employ two approaches of source depth estimation. One method employs downward continuation of positive and negative sources from mapping level into the subsurface to extrapolate the depth to magnetization. With this approach, source depths generally range from 80 ± 20 km in Terra Meridiani and 65 ± 25 km in Terra Sirenum. A graphical approach uses the contour map of surface magnetization to estimate depths ranging from 125 km for thick sources in Terra Meridiani and from 82 km for thick sources in Terra Sirenum. These depths require a low (≤∼20 mW/m2) Martian heat flux to permit magnetite, hematite, and/or pyrrhotite (although limited) as carriers through 100 km or more. The upcoming InSight mission will provide invaluable seismic constraints on both crustal and core structure, in addition to the first Martian heat flow measurements that will constrain magnetization.

Magnetic mineralogy of the Mercurian lithosphere

AbstractMercury and Earth are the only inner solar system planets with active, internally generated dynamo magnetic fields. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission recently detected magnetic fields on Mercury that are consistent with lithospheric magnetization. We investigate the physical and chemical environment of Mercury's lithosphere, past and present, to establish the conditions under which magnetization may have been acquired and modified. Three factors are particularly crucial to the determination of crustal composition and iron mineralogy: redox conditions in the planet's crust and mantle, the iron content of the lithosphere, and, for any remanent magnetization, the temperature profile of the lithosphere and its evolution over time. We explore potential mechanisms for remanence acquisition and alteration on Mercury, whose surface environment is both hot and highly reducing. The long‐term thermal history of Mercury's crust plays an important role in the longevity of any remanent crustal magnetization, which may be subject to remagnetization through thermal, viscous, and shock mechanisms. This thermal and compositional framework is used both to constrain plausible candidate minerals that could carry magnetic remanence on Mercury and to evaluate their capacity to acquire and retain sufficient magnetization to be detectable from satellite orbit. We propose that iron metal and its alloys are likely to be the dominant contributors to induced and remanent magnetization in Mercury's lithosphere, with additional contributions from iron silicides, sulfides, and carbides.

Crustal magnetization and the subseafloor structure of the ASHES vent field, Axial Seamount, Juan de Fuca Ridge: Implications for the investigation of hydrothermal sites

AbstractHigh‐resolution geophysical data have been collected using the Autonomous Underwater Vehicle (AUV) Sentry over the ASHES (Axial Seamount Hydrothermal Emission Study) high‐temperature (~348°C) vent field at Axial Seamount, on the Juan de Fuca Ridge. Multiple surveys were performed on a 3‐D grid at different altitudes above the seafloor, providing an unprecedented view of magnetic data resolution as a function of altitude above the seafloor. Magnetic data derived near the seafloor show that the ASHES field is characterized by a zone of low magnetization, which can be explained by hydrothermal alteration of the host volcanic rocks. Surface manifestations of hydrothermal activity at the ASHES vent field are likely controlled by a combination of local faults and fractures and different lava morphologies near the seafloor. Three‐dimensional inversion of the magnetic data provides evidence of a vertical, pipe‐like upflow zone of the hydrothermal fluids with a vertical extent of ~100 m.

Investigation of a marine magnetic polarity reversal boundary in cross section at the northern boundary of the Kane Megamullion, Mid‐Atlantic Ridge, 23°40′N

AbstractNear‐bottom magnetic field measurements made by the submersible Nautile during the 1992 Kanaut Expedition define the cross‐sectional geometry of magnetic polarity reversal boundaries and the vertical variation of crustal magnetization in lower oceanic crust exposed along the Kane Transform Fault (TF) at the northern boundary of the Kane Megamullion (KMM). The KMM exposes lower crust and upper mantle rocks on a low‐angle normal fault that was active between 3.3 Ma and 2.1 Ma. The geometry of the polarity boundaries is estimated from an inversion of the submarine magnetic data for crustal magnetization. In general, the polarity boundaries dip away from the ridge axis along the Kane TF scarp, with a west dipping angle of ~45° in the shallow (<1 km) crust and <20° in the deeper crust. The existence of the magnetic polarity boundaries (e.g., C2r.2r/C2An.1n, ~2.581 Ma) indicates that the lower crustal gabbros and upper mantle serpentinized peridotites are able to record a coherent magnetic signal. Our results support the conclusion of Williams (2007) that the lower crust cools through the Curie temperature of magnetite to become magnetic, with the polarity boundaries representing both frozen isotherms and isochrons. We also test the effects of the rotation of this isotherm structure and/or footwall rotation and find that the magnetic polarity boundary geometry is not sensitive to these directional changes.

Geological features of the northeastern Canadian Arctic margin revealed from analysis of potential field data

The northeastern Canadian Arctic margin is bordered to the north by Alpha Ridge, a dominantly magmatic complex within the Amerasia Basin of the Arctic Ocean, which forms part of the High Arctic Large Igneous Province (HALIP). The characteristics of the gravity and magnetic anomaly fields change notably along the Arctic margin, with two main segments recognised. Aeromagnetic and gravity data in the transition zone between these contrasting domains of the Canadian Arctic margin are analysed here in detail. Results obtained using a variety of edge enhancement (derivative) methods highlight several magnetic domains and a major offshore sedimentary basin as well as some known and a number of previously unknown tectonic and magmatic elements. A magmatic intrusion distribution map derived from the edge enhanced magnetic anomaly maps reveals that magmatic rocks are much more widespread in the relatively shallow subsurface than implied by surface geological mapping. Magmatic intrusions (mainly dykes) and other geological structures have NW–SE, NE–SW and N–S major trends. Broad gravity and pseudogravity lows across most of the Sverdrup Basin region are due to thick, less dense sedimentary succession and low magnetised crust. Magnetic and pseudogravity highs observed over Alpha Ridge indicate high crustal magnetisation associated with the occurrence of extensive and voluminous crustal magmatic bodies. Absence of these volcanic and intrusive rocks in the imaged sedimentary basin beneath the northeast Canadian Arctic margin region suggests that the basin probably formed after the cessation of HALIP magmatism.

Origin of magnetic highs at ultramafic hosted hydrothermal systems: Insights from the Yokoniwa site of Central Indian Ridge

High-resolution vector magnetic measurements were performed on an inactive ultramafic-hosted hydrothermal vent field, called Yokoniwa Hydrothermal Field (YHF), using a deep-sea manned submersible Shinkai6500 and an autonomous underwater vehicle r2D4. The YHF has developed at a non-transform offset massif of the Central Indian Ridge. Dead chimneys were widely observed around the YHF along with a very weak venting of low-temperature fluids so that hydrothermal activity of the YHF was almost finished. The distribution of crustal magnetization from the magnetic anomaly revealed that the YHF is associated with enhanced magnetization, as seen at the ultramafic-hosted Rainbow and Ashadze-1 hydrothermal sites of the Mid-Atlantic Ridge. The results of rock magnetic analysis on seafloor rock samples (including basalt, dolerite, gabbro, serpentinized peridotite, and hydrothermal sulfide) showed that only highly serpentinized peridotite carries high magnetic susceptibility and that the natural remanent magnetization intensity can explain the high magnetization of Yokoniwa. These observations reflect abundant and strongly magnetized magnetite grains within the highly serpentinized peridotite. Comparisons with the Rainbow and Ashadze-1 suggest that in ultramafic-hosted hydrothermal systems, strongly magnetized magnetite and pyrrhotite form during the progression of hydrothermal alteration of peridotite. After the completion of serpentinization and production of hydrogen, pyrrhotites convert into pyrite or nonmagnetic iron sulfides, which considerably reduces their levels of magnetization. Our results revealed origins of the magnetic high and the development of subsurface chemical processes in ultramafic-hosted hydrothermal systems. Furthermore, the results highlight the use of near-seafloor magnetic field measurements as a powerful tool for detecting and characterizing seafloor hydrothermal systems.

Magnetic Signatures and Curie Surface Trend Across an Arc–Continent Collision Zone: An Example from Central Philippines

Ground and aeromagnetic data are combined to characterize the onshore and offshore magnetic properties of the central Philippines, whose tectonic setting is complicated by opposing subduction zones, large-scale strike-slip faulting and arc–continent collision. The striking difference between the magnetic signatures of the islands with established continental affinity and those of the islands belonging to the island arc terrane is observed. Negative magnetic anomalies are registered over the continental terrane, while positive magnetic anomalies are observed over the Philippine Mobile Belt. Several linear features in the magnetic anomaly map coincide with the trace of the Philippine Fault and its splays. Power spectral analysis of the magnetic data reveals that the Curie depth across the central Philippines varies. The deepest point of the magnetic crust is beneath Mindoro Island at 32 km. The Curie surface shallows toward the east: the Curie surface is 21 km deep between the islands of Sibuyan and Masbate, and 18 km deep at the junction of Buruanga Peninsula and Panay Island. The shallowest Curie surface (18 km) coincides with the boundary of the arc–continent collision, signifying the obduction of mantle rocks over the continental basement. Comparison of the calculated Curie depth with recent crustal thickness models reveals the same eastwards thinning trend and range of depths. The coincidence of the magnetic boundary and the density boundary may support the existence of a compositional boundary that reflects the crust–mantle interface.

The effect of hydrostatic pressure up to 1.61 GPa on the Morin transition of hematite‐bearing rocks: Implications for planetary crustal magnetization

AbstractWe present new experimental data on the dependence of the Morin transition temperature (TM) on hydrostatic pressure up to 1.61 GPa, obtained on a well‐characterized multidomain hematite‐bearing sample from a banded iron formation. We used a nonmagnetic high‐pressure cell for pressure application and a Superconducting Quantum Interference Device magnetometer to measure the isothermal remanent magnetization (IRM) under pressure on warming from 243 K to room temperature (T0). IRM imparted at T0 under pressure in 270 mT magnetic field (IRM270mT) is not recovered after a cooling‐warming cycle. Memory effect under pressure was quantified as IRM recovery decrease of 10%/GPa. TM, determined on warming, reaches T0 under hydrostatic pressure 1.38–1.61 GPa. The pressure dependence of TM up to 1.61 GPa is positive and essentially linear with a slope dTM/dP = (25 ± 2) K/GPa. This estimate is more precise than previous ones and allows quantifying the effect of a pressure wave on the upper crust magnetization, with special emphasis on Mars.

Kinetic simulations of kilometer-scale mini-magnetosphere formation on the Moon

Kinetic simulations are used to examine the solar wind's interaction with a 3 km wide region of strong crustal dipole magnetization on the Moon. In contrast with recent hybrid and implicit particle-in-cell simulations of magnetic anomalies that have aimed to resolve electric fields over several tens of kilometers, kinetic simulations reveal a much smaller scale regime in which magnetically driven ion-electron separation can generate a kV potential difference over a height of less than 200 m. The resulting electric field structure varies considerably between dawn and noon (when the solar wind flows, respectively, horizontally across the surface and vertically down from above) and is strong enough to reflect some ions back into space, consistent with spacecraft observations. Ion velocity and energy distributions are extracted near the surface and are used to derive maps of ion flux and impact energy, and the effects on sputtering and defect formation within the regolith are discussed. However, considerable uncertainty remains in how the surface ion flux evolves throughout a lunar day and how the plasma-surface-magnetic field interaction changes with respect to different magnetic topologies.

Influence of redox conditions on the intensity of Mars crustal magnetic anomalies

AbstractWe evaluate the relationship between the intensity of remanent magnetization and fO2 in natural and synthetic Mars meteorites. The olivine‐phyric shergottite meteorite Yamato 980459 (Y‐980459) and a sulfur‐free synthetic analog (Y‐98*) of identical major element composition were analyzed to explore the rock magnetic and remanence properties of a basalt crystallized from a primitive melt, and to explore the role of magmatic and alteration environment fO2 on Mars crustal anomalies. The reducing conditions under which Y‐980459 is estimated to have formed (QFM‐2.5; Shearer et al. 2006) were replicated during the synthesis of Y‐98*. Y‐980459 contains pyrrhotite and chromite. Chromite is the only magnetic phase in Y‐98*. The remanence‐carrying capacity of Y‐980459 is comparable to other shergottites that formed in the fO2 range of QFM‐3 to QFM‐1. The remanence‐carrying capacity of these low fO2 basalts is 1–2 orders of magnitude too weak to account for the intense crustal anomalies observed in Mars's southern cratered highlands. Moderately oxidizing conditions of >QFM‐1, which are more commonly observed in nakhlites and Noachian breccias, are key to generating either a primary igneous assemblage or secondary alteration assemblage capable of acquiring an intense remanent magnetization, regardless of the basalt character or thermal history. This suggests that if igneous rocks are responsible for the intensely magnetized crust, these oxidizing conditions must have existed in the magmatic plumbing systems of early Mars or must have existed in the crust during secondary processes that led to acquisition of a chemical remanent magnetization.

Maximum depth of magnetisation of Australia, its uncertainty, and implications for Curie depth

The Curie depth is the depth at which the crust and uppermost mantle cease to be ferromagnetic or ferromagnetic, the main cause of crustal magnetism, due to the action of geothermal effects. One method to estimate the Curie depth for Australia is to map the base of magnetisation derived from observations of magnetic intensity. We have used a nonlinear direct sampling inverse technique to fully explore the parameter space of a fractal forward model of magnetisation. This produces an ensemble of models that allow us to produce maps of both the maximum depth of magnetisation and its uncertainty for Australia. The base of magnetisation varies significantly across the continent, between 10 and 70km depth, with an uncertainty of 7–10km. The variations in magnetisation depth conform with the boundaries of geological provinces due to their differing magnetic properties: In general, cratons and older provinces generally show a deeper base of magnetisation results and hence may be inferred to have deeper Curie depths, reflecting that these areas are on the whole cooler. We also find general agreement in our results with known geothermal anomalies.

A 3D model of crustal magnetization at the Pinacate Volcanic Field, NW Sonora, Mexico

The Pinacate Volcanic Field (PVF) is located near the western border of the southern Basin and Range province, in the State of Sonora NW Mexico, and within the Gulf of California Extensional Province. This volcanic field contains the shield volcano Santa Clara, which mainly consists of basaltic to trachytic volcanic rocks, and reaches an altitude of ~1200m. The PVF disrupts a series of discontinuous ranges of low topographic relief aligned in a NW direction, which consist mainly of Proterozoic metamorphic rocks and Proterozoic through Paleogene granitoids. The PVF covers an area of approximately 60 by 55km, and includes more than 400 well-preserved cinder cones and vents and eight maar craters. It was active from about 1.7Ma until about 13ka. We have used the ages and magnetic polarities of the volcanic rocks, along with mapped magnetic anomalies and their inverse modeling to determine that the Pinacate Volcanic Field was formed during two volcanic episodes. The oldest one built the Santa Clara shield volcano of basaltic and trachytic composition, and occurred during the geomagnetic Matuyama Chron of reverse polarity, which also includes the normal polarity Jaramillo and Olduvai Subchrons, thus imprinting both normal and reverse magnetization in the volcanic products. The younger Pinacate series of basaltic composition represents monogenetic volcanic activity that extends all around the PVF and occurred during the subsequent geomagnetic Brunhes Chron of normal polarity. Magnetic anomalies toward the north of the Santa Clara volcano are the most intense in the PVF, and their inverse modeling indicates the presence of a large subsurface body magnetized in the present direction of the geomagnetic field. This suggests that the magma chambers at depth cooled below the Curie temperature during the Brunhes Chron.

High‐resolution magnetic signature of active hydrothermal systems in the back‐arc spreading region of the southern Mariana Trough

AbstractHigh‐resolution vector magnetic measurements were performed on five hydrothermal vent fields of the back‐arc spreading region of the southern Mariana Trough using Shinkai 6500, a deep‐sea manned submersible. A new 3‐D forward scheme was applied that exploits the surrounding bathymetry and varying altitudes of the submersible to estimate absolute crustal magnetization. The results revealed that magnetic‐anomaly‐derived absolute magnetizations show a reasonable correlation with natural remanent magnetizations of rock samples collected from the seafloor of the same region. The distribution of magnetic‐anomaly‐derived absolute magnetization suggests that all five andesite‐hosted hydrothermal fields are associated with a lack of magnetization, as is generally observed at basalt‐hosted hydrothermal sites. Furthermore, both the Pika and Urashima sites were found to have their own distinct low‐magnetization zones, which could not be distinguished in magnetic anomaly data collected at higher altitudes by autonomous underwater vehicle due to their limited extension. The spatial extent of the resulting low magnetization is approximately 10 times wider at off‐axis sites than at on‐axis sites, possibly reflecting larger accumulations of nonmagnetic sulfides, stockwork zones, and/or alteration zones at the off‐axis sites.

Determination of block rotations and the Curie Point Depths of magnetic sources along the NW–SE-trending Sülüklü–Cihanbeyli-Gölören and Şereflikoçhisar-Aksaray Fault Zones, Central Anatolia, Turkey

The Şereflikoçhisar-Aksaray Fault (SAF) and Sülüklü–Cihanbeyli–Gölören Fault (SCGF) zones are located at the eastern and the western margins of the Salt Lake Basin-SLB (Tuzgölü Basin), central Anatolia, Turkey. These fault zones display intense magnetic anomalies along their trends and the positive amplitudes of these anomalies may reach up to 650 nT. In this paper, the body magnetisation directions are estimated from the selected five different magnetic anomalies with positive and negative peaks along these fault zones. Results of calculations indicated that local clockwise rotations are in the range of 34°E and 70°E. These magnetic anomalies were mainly created by the buried intrusive rocks. Locations of buried causative sources along the fault zones were determined from the analytic signal transformation of magnetic anomaly data. Curie Point Depths (CPDs) are also estimated from the aeromagnetic data to determine the thermal structure of the study area and they are correlated with the SAF and SCGF zones. It was found that the depth of magnetic crust varies from 11 to 22 km, consistent with the tectonic fault zones in the study area. The average value was determined as 16.7 km. These depths were correlated with the deep seismic reflection data and depths obtained from two windows in the seismic section, GTRS-87-801 are consistent with the CPDs. Shallow CPDs of magnetic sources are located along the SAF and SCGF to the east and west of the Lake Tuzgölü (Salt Lake) and it may be suggested that magnetic sources are probably located at the upper part of the crust.

Curie Point Depths and Heat Flow of Eastern Anatolia (Turkey)

The bottom of the magnetized crust determined from the spectral analysis of magnetic anomalies is generally interpreted as the level of the Curie point isotherm. The spectral analysis method to estimate the depth extent of magnetic sources was applied to the magnetic anomalies of Eastern Anatolia and compared with the tectonic regime and heat flow data in the region. The Curie point depth of eastern Anatolia ranges from 6 to 24 km. The computed Curie depths and the heat flow values derived by using Curie depths are consistent with the geological and the seismological findings of the region. Shallow Curie point depths and high heat flow values are seen between the Bitlis Suture Zone and the pontides. At the same time, this region might contain a thinner crust than expected.

Could giant basin-forming impacts have killed Martian dynamo?

The observed strong remanent crustal magnetization at the surface of Mars suggests an active dynamo in the past and ceased to exist around early to middle Noachian era, estimated by examining remagnetization strengths in extant and buried impact basins. We investigate whether the Martian dynamo could have been killed by these large basin-forming impacts, via numerical simulation of subcritical dynamos with impact-induced thermal heterogeneity across the core-mantle boundary. We find that subcritical dynamos are prone to the impacts centered on locations within 30° of the equator but can easily survive those at higher latitudes. Our results further suggest that magnetic timing places a strong constraint on postimpact polar reorientation, e.g., a minimum 16° polar reorientation is needed if Utopia is the dynamo killer.